PrePost Grammar checker is totally free online tool. it checks spellings and grammar mistakes in your article, essay on just one click. Red underline indicates spell error while green under line indicates grammar error in your content. you can also change the world and sentence by clicking on the error.
This grammar checking tool is also available for wordpress users, they can find out mistakes in there blog post directly from the admin panel of the wordpress.
Rich text grammar checking feature is currently only avialable for wordpress plugin.